1. Field of the Invention
This invention relates generally to any controller for a power supply that utilizes a compensator to shape the response of the control loop to external disturbances, and more particularly to a control means for calculating the dynamic impedance of a DC magnetron based process.
2. Brief Description of the Prior Art
In plasma processing for the manufacture of thin films for integrated circuits, flat panel displays, glass coatings, etc a fast controller is required to effectively control the power delivered to a wide range of plasma processes. Controllers are designed accounting for the control to output transfer function of the power supply. The transfer function of the power supply depends on the dynamic impedance of the load. The primary function of a controller for a power supply is to achieve and maintain any commanded control signal. The controller is designed accounting for the control to output, and line to output transfer functions of the power supply. The dependence of the controller on load impedance may be in the form of DC gain, or the location of a pole or a zero in the transfer function of the power supply. Any change in the output impedance can significantly influence the performance of the control loop and sometimes even catastrophically by making a previously stable system unstable. As shown in FIG. 1 the dynamic impedance is positive for curves A and B and negative for curve C. The voltage and resistance are a function of the operating point and can change with time. FIG. 2 illustrates how depending on the process and plasma characteristics the plasma load can be modeled as a voltage source in series with a resistance. In the cases where variations in the transfer function due to a change in the dynamic impedance may be limited to the DC gain and could be easily compensated with analog circuitry or digital gain blocks, it is performed by a priori analysis of the transfer functions and implementing lookup tables for different load conditions.
Some controllers utilize DC current and the DC voltage to calculate the DC impedance of the plasma. However, this method has a disadvantage, since it assumes that the plasma represents a load that is only an impedance in nature. In the case of model plasma as shown in FIG. 2 then an approach is to use an empirical value of the plasma voltage and subtract this from the DC voltage and then use this to calculate the actual dynamic impedance of the plasma. This approach has a disadvantage because there are wide ranges of internal voltages that even the same plasma can exhibit.
For instance, U.S. Pat. No. 5,543,689 issued to Ryusuke Ohta et al discloses a high frequency power source wherein the controller has a memory for storing initial plasma characteristic data and plasma gain, a comparable operation section for calculating control target data from initial plasma characteristic data and detected power data and computing the control gain data from the difference between the control target data and the power control signal data. The control target data is derived by subtracting the initial plasma data from the detected plasma data. However, the process described relies upon an alarm for manual adjustment of the control signal. Such a system has a slow response time and may cause the process to shut down before correction can be made.
These types of control schemes are undesirable because 1) estimation of the plasma resistance is extremely difficult; 2) implementation of the compensation is limited to the DC gain; 3) they are not continuous and dependence may not be monotonic with respect to the load impedance; and 4) the variation in the poles and zeros of the transfer function with the load impedance may still degrade the performance of the system and in some instances may also cause the system to go unstable. Also, faster loop speeds require ability to compensate for load and create transfer functions to create a fairly wide-band system.
It would be desirable if there were provided a controller for a DC power supply that utilized a fast control loop which works for a wide range of plasma processes. It would also be desirable if there were provided method of control which enables adaptive and non-linear control by estimating dynamic impedance of the load.